The present invention relates to a battery capacity detection system and, more particularly, to a battery capacity detection system with a chargeable capacity calculating function using a temperature sensor, for correcting a voltage in a full charge state by using an actually detected temperature and obtaining a chargeable capacity at the time point.
In recent years, an accurate prediction of a state of charge (SOC, also called dischargeable capacity) of a battery as a power source mounted on an electric vehicle has been being attempted.
Japanese Patent Application Laid-Open Publication No. 8-278351 discloses a battery SOC measuring apparatus.
In the battery SOC measuring apparatus, since calculation is easy, a battery controller calculates the SOC of a battery immediately after turning on an ignition by the following equation and displays the result. EQU SOC=(Vn.sup.2 -Ve.sup.2)/(Vs.sup.2 -Ve.sup.2).times.100(%)
where, Vn denotes an estimated voltage of the battery, Ve denotes a discharge end voltage (a fixed value) and Vs is a voltage of the battery in a fully charged state.
The voltage Vs in the fully charged state is a voltage when the battery is fully charged (substantially 100%) at the temperature of, for example, 20.degree. C. in an initial state where degradation of the battery is not progressed yet. That is, the voltage Vs is a fixed value.
The estimated voltage Vn of the battery is obtained as follows. First, a predetermined number of values of each of a discharge current from the battery and a terminal voltage are collected and averaged. When a predetermined number of the averaged data is collected, a correlation coefficient (r) of the data is obtained. When the correlation coefficient (r) shows a strong negative correlation, a regression line (also called approximation line) is obtained by the method of least squares. From the approximation line Y (Y=aX+b) and a reference current Io, the present battery estimated voltage Vn is calculated.
The present SOC during driving is derived by the equation by using the estimated voltage Vn and the like and is properly displayed.
That is, in the battery SOC measuring apparatus, the SOC can be known without measuring the temperature.
In what is called a hybrid vehicle as well, the battery SOC measuring apparatus is used. Some battery SOC measuring apparatuses each for use in a hybrid vehicle obtain a regenerative energy to charge the battery. There is a case such that the SOC of the battery during driving is calculated, a chargeable capacity is obtained from the difference between the SOC and the SOC in the full charge state and, if it is chargeable, the battery is charged.